1. Technical Field
The present invention relates to a method of fabricating a printed circuit board.
2. Description of the Related Art
With mobile electronic equipment rapidly trending towards lighter, smaller, and thinner products that provide higher performance, and with the demands for LSI packages having higher speeds and higher densities, there is currently a demand also for higher-density build-up boards. Currently, in a flip chip board, the mainstream is to use a SAP (semi-additive process) in forming the circuits, in order to increase the wiring density of the board. However, there is ongoing research on methods other than SAP methods, for conveniently forming fine-line circuits.
One possible drawback of conventional circuit forming methods may be that, since a circuit made from copper is formed on the insulation layer, the portion attached to the insulation layer is only the bottom surface of the circuit. Thus, when forming a fine-line circuit, undercuts may occur at the attachment portions between the circuit and the resin, whereby the fine-line circuit may be peeled off. Such peeling off would cause the entire board to be defective.
To resolve such a problem, a technology is being researched of forming a trench in the insulation layer, by imprinting or by laser processing, and then filling a conductive material inside the trench.
Imprinting may involve forming relieve and intaglio patterns in the surface of a mold to process a circuit pattern, and afterwards transcribing this into an insulation resin to form the trench in the insulation resin. Then, a conductive material may be filled in the trench, such as by filling with metal paste or performing metal plating, to form the circuit. With laser processing, the trench may be formed in the surface of a stacked insulation layer by laser ablation, and the circuit may be formed in a manner similar to that for the imprinting method, by filling with metal paste or by plating.
However, in the imprinting method, the mold is generally expensive, and there is much difficulty in separating the mold from the insulation layer after transcribing the patterns on the mold. Also, curing the resin while the mold is being pressed may cause the pressing to be very time-consuming, and may cause lowered productivity. With the laser processing, not only is the equipment itself highly expensive, but also the method may require expensive aluminum masks or dielectric masks for forming the circuit.
Another important factor in the fabrication of a printed circuit board is the method of interlayer connection.
For the interlayer connection, conventional methods use mechanical hole processing and plating, where the points of importance are on the minuteness and accuracy of the mechanical processing and on how efficiently the plating for interlayer connection is performed in the hole formed by the mechanical processing. There is also a technology being developed and used, of processing blind via holes using a laser, instead of mechanical hole processing, and furthermore, there are developments and productions in B2it (buried bump interconnection technology) and NMBI (Neo Manhattan bump interconnection) technology.
However, with mechanical hole processing, the abrasion of the bit may cause increased costs, and with B2it, the use of paste for interlayer connection renders B2it as yet unreliable for use in semiconductor substrates, and there is a limit to how minute the holes can be made. Also, since NMBI physically connects copper with copper, there may be problems in contact reliability.